Removable partial dental prostheses are employed to restore one or more, but not all, of the natural teeth of a patient. The primary objectives of a properly designed dental prosthesis include the preservation of remaining teeth, along with hard and soft oral tissue, the restoration of oral function, and the restoration of dental and facial aesthetics.
Removable partial dental prostheses are commonly formed with a base or saddle which rests on the oral mucosa in an edentulous space and is coupled by connector means to one or more abutment teeth, that is, a tooth which abuts or is adjacent to the edentulous space.
Perhaps one of the most common forms of partial dental prosthesis is the distal extension removable partial denture which is used to replace posterior teeth with artificial teeth on one or both sides of the mouth. Distal extensions are commonly secured to an abutment tooth by clasps, which are mounted on the abutment tooth to provide for stabilization and retention of the prosthesis, and a connector, which extends between the clasp and the base of the distal extension. The base or saddle of the distal extension rests on the alveolar ridge.
Since the mucosal covering over the alevolar bone can be compressed or displaced when loaded, for example, during mastication, a rigid connection between the abutment tooth clasp and the denture base results in substantial lateral loading of the abutment tooth. Thus, as the user of the denture chews a bolus of food, compression and resilient rebound of the mucosa on the alveolar ridge will cause the denture base to be vertically reciprocated, which reciprocation is in turn transferred to lateral displacement of the abutment tooth by the connection between the base and the clasp mounted on the abutment tooth.
Since natural teeth are supported by periodontal fibers which are well suited for support of compression loading but not well suited for repeated lateral displacement, the result of repeated lateral displacement is that the prosthesis gradually breaks down the periodontal fibers supporting the abutment tooth, with the further result that the abutment tooth and supporting bone are damaged. The abutment tooth will have to be removed and the denture supported from the next abutment tooth. Since the process repeats itself, there is a gradual breakdown of the natural teeth as a result of using a dental prosthesis which is rigidly secured to abutment teeth.
This problem is further exacerbated by the fact that during normal mastication, the loading forces are not simply in the vertical direction. During the chewing cycle the mandible moves diagonally downward and forward, then to the side that contains the bolus of food, then backwardly, and finally forward to complete the cycle. The chewing path, therefore, is eliptical, and the forces on any denture are multi-directional. Thus, the lateral or rocking force on the abutment tooth as a result of compression of the alveovar ridge mucosa by the dental prosthesis is also multi-directional, and the breakdown process of the periodontal membrane and fibers supporting the abutment tooth is accordingly more rapid than would be the case if the chewing action were merely vertical in direction.
Prosthodontists have long sought the most suitable design for distal extension removable partial dentures. Such designs require a balance between stress-free retention of the denture in the mouth and physiological integrity necessary for the reproduction of natural oral function. Dental journals have long reported the relatively high failure rate of distal extension partial dentures as a result of lateral stressing of abutment teeth, and great attention has been directed to the torque and stress forces applied to the abutment tooth and to the mucosa on the alveolar ridge, which also tends to become traumatized and destroyed through repeated compression.
In recent years attempts have been made to redirect and reduce the stress on the abutment tooth by providing a hinged connector between the abutment tooth and the base of the prosthesis. The proposed function of the hinge is to direct vertical force away from the abutment tooth and transfer that force to the alveolar ridge. Thus, a hinge connector in which the pivotal axis is horizontally oriented will permit downward displacement of the base without tilting or pulling the abutment tooth laterally to any substantial degree. Actually the hinge movement is arcuate and mastication, even in the vertical direction, can cause some rearward displacement of the abutment tooth.
The problem with hinged distal extension removable partials, however, is that the chewing action is not merely in the vertical direction, as above noted. Accordingly, the natural eliptical chewing action causes displacement of the prosthesis in directions which cannot be accommodated by a hinge connector with a horizontal axis. The abutment tooth, therefore, will still be rocked during mastication to such an excessive degree as to cause breakdown of the periodontal membrane. Moreover, it should be noted that lateral forces can be induced in directions which a hinge cannot accommodate by factors other than the normal eliptical chewing movement, such as off-center contact with the bolus of food, inaccurate set up of the artificial teeth, inaccurate bite registration, and natural occlusal diseases such as Bruxism.
The use of hinged removable partial denture prostheses, therefore, has been limited in nature, and the problems sought to be overcome by the structure still remain. The marginal improvement in dental performance of hinged partial prosthesis, therefore, has not justified the increased complexity of the prosthesis construction, and most prosthodontists still employ the rigidly connected distal extension removable partial denture.